This invention relates to combustors, and more specifically, to hybrid combustors for providing a substantially uniform fuel and air mixture.
Combustors for gas turbines typically comprise a combustion chamber together with burners, igniters, and fuel injection devices. Combustors for gas turbines have traditionally operated in a non-premixed mode in which a fuel (e.g., natural gas) and an oxidant (e.g., air) are completely separated as the reactants enter the flame. In general, non-premixed combustors are stable over a wide range of operating conditions and at low fuel-air ratios. A drawback of non-premixed combustors, however, is that high temperatures in the reaction zone lead to increased production of nitrogen oxides (NOx).
In premixed combustors, the fuel and the oxidant are completely premixed before combustion. The production of NOx in premixed flames is minimized because localized high temperatures in the reaction zone are avoided. A drawback of premixed combustors is that at low loads, premixed combustors produce higher levels of carbon monoxide (CO) and unburned hydrocarbons (UHCs) and are also not as stable compared to non-premixed combustors. Although the flame stability in premixed combustors can be improved through mechanical and aerodynamic means (e.g., fuel nozzles having a bluff body with a broad flattened surface for causing recirculation of the flow of the fuel and air mixture having swirlers), premixed combustors generally lack the stability of non-premixed combustors.
An approach for stabilizing premixed combustors is the application of a catalyst in the combustor to initiate and promote gas phase combustion, which combustion has been referred to sometimes as xe2x80x9ccatalytic combustionxe2x80x9d, catalytically stabilized combustion, or xe2x80x9ccatalytically stabilized thermal combustion.xe2x80x9d A drawback of catalytic combustors is that their maximum operating temperature may be limited by the thermal stability of the catalytic materials or the mechanical supports. Another drawback is that non-uniformities in the fuel-air mixture, for example, from a fuel nozzle, result in areas of localized overheating if the fuel-air mixture is too rich, or areas of low catalyst activity if the fuel-air mixture is too lean.
Therefore, there is a need for hybrid combustors which provide stable high and low levels of operation while minimizing emissions of NOx at high levels of operation and minimizing emissions of CO or UHCs at low levels of operation. In addition, there is a need for fuel nozzles for providing a substantially uniform fuel and air mixture.
A hybrid combustor, for providing stable high and low levels of operation while minimizing emissions of NOx, CO, and UHCs, includes a casing having a chamber, a catalytic combustor disposed in the chamber, and a non-premixed combustor disposed in the chamber. The hybrid combustor may comprise a fuel nozzle comprising a casing having a chamber, and a body supportable in the chamber to define a passageway between the body and the casing. The passageway has an inlet for receiving a stream of air and an outlet for discharging a stream of fuel and air, and the body includes a tapering downstream portion. Desirably, flow separation of the fuel and air mixture from the body (i.e., recirculation of the fuel and air mixture in the passageway or chamber) is inhibited whereby a generally uniform fuel and air mixture is provided.